1. Field of the Invention
This invention relates to III-V nitride homoepitaxial material films of improved MOVPE epitaxial quality, formed on corresponding free-standing substrates, as well as optical-electronic and electronic devices and device precursor structures comprising such films.
2. Description of the Related Art
(Al,In,Ga)N (which term as used herein refers inclusively and alternatively to each of individual nitrides containing one or more of Al, In and Ga, thereby alternatively encompassing each of AlN, AlxIn1-xN (or AlInN), AlxGa1-xN (or AlGaN), AlxInyGa1-x-yN (or AlInGaN), InN, InyGa1-yN (or InGaN) and GaN where 0≦x≦1 and 0≦y≦1, as well as mixtures thereof and doped layers (n-type or p-type) or remaining undoped) has been extensively studied with respect to its epitaxial layer growth on heavily lattice-mismatched substrates such as sapphire and SiC.
A primary reason for the pervasive character of such research is that free-standing (FS), coefficient of thermal expansion (CTE)—matched, and lattice-matched GaN substrates of suitable quality and size are unavailable.
Without homoepitaxial or native substrates, misfit dislocations will form due to lattice mismatch at the epitaxy-substrate interface, and cracking and bowing will occur due to the CTE mismatch, thereby limiting the quality of the epi and device layers. The epitaxial layer quality on these non-optimal substrates (e.g., sapphire or SiC) is of reasonable quality for simple electronic devices if complicated interlayer techniques are used.
Typically, to make higher quality devices, very difficult and complicated overgrowth techniques such as ELOG (epitaxial lateral overgrowth) or LEO (lateral epitaxial overgrowth) or Pendeo-epitaxy are employed, but the resulting material is non-uniform in morphology and crystalline quality. Further, the resulting material typically has a high carrier concentration due to impurity incorporation from the masking material. Such overgrowth techniques employ the use of a masking material such as SiO2 to inhibit growth in certain areas on the substrate material. The epitaxial material then grows between the masked region and then laterally over the masking material, thereby reducing dislocation propagation in the laterally grown area.
The lack of a suitable quality lattice-matched (Al,In,Ga)N substrate has impeded (Al,In,Ga)N device developers from realizing the full potential of the (Al,In,Ga)N device capabilities and slowed the development of this material system. The complexity and difficulties attendant the lateral overgrowth techniques have prevented such approach from being satisfactorily commercially used.
A small amount of work has been done to produce nitride substrates and an even smaller amount of epitaxial layer growth has been done on the limited amount of GaN material produced.
As a background to discussion of the problems with GaN epilayer growth on FS GaN, techniques for producing free-standing GaN are described below. The ensuing discussion also highlights how the properties of some of the substrates have inhibited the development of a suitable epitaxial process.
Substrate Production
Potential methods for producing lattice matched or nearly lattice matched substrates superior to sapphire and SiC that have been developed to date include high pressure GaN crystal growth, AlN bulk growth, lithium aluminate (LAO), lithium gallate (LGO), thick (>100 um) HVPE GaN and lift-off, and HVPE GaN boule growth, as discussed more fully below.
High Pressure Crystal Growth
High pressure crystal growth has been successful in producing small platelets (<20 mm diameter and <1-2 mm thick) of less than 300 square millimeters area of single crystal GaN but the GaN crystals have several problems. This technique produces small platelets and the scalability is difficult and the cost of the process is quite large compared to other alternatives. Further, dopant and conductivity control of the crystal is very difficult due to the technique. Another disadvantage is that high unintentional impurity levels are present in the crystal including oxygen, which make the substrates conductive. These high levels of impurities limit the frequency range of devices produced on the substrates due to parasitic capacitances between device layers and charge in the substrate and may inhibit epitaxy nucleation on the substrate at sufficiently high impurity concentrations.
AlN (or GaN) Substrate Formation Via Sublimation and Re-Condensation
The production of bulk AlN by sublimation and re-condensation technique is being performed to produce suitable, high quality, nearly lattice-matched (2.5% difference from GaN) substrates for GaN epitaxial growth. Currently, the boule diameter is limited to 13 millimeters, severely limiting the production of lost cost, high volume devices.
Another issue with these substrates is the extremely high oxygen level, on the order of parts per million (ppm), which will likely reduce the thermal conductivity of the substrates, making them less advantageous for high frequency, high power devices.
In addition to affecting the thermal conductivity, the high impurity incorporation in these substrates inhibits the production of controlled electrical conductivity type substrates, namely p-type substrates. These substrates are difficult to dope heavily by conventional techniques, making them less advantageous for vertical opto-electronic device structures. In the case of AlN substrates, the substrate and associated devices are disadvantaged by high ionization or activation energy of acceptors and donors in the crystal, as compared to GaN substrates.
Lithium Aluminate (LAO) and Lithium Gallate (LGO)
LAO and LGO are closely lattice-matched substrates (compared to SiC and sapphire) and are available in reasonable quality and size, however, several issues exist that prevent their applicability to the GaN material system. Most importantly, LAO and LGO materials suffer from low decomposition temperatures preventing them from being easily used for GaN growth at typical growth temperatures. Li and Ga desorption and diffusion from the substrate into the epitaxial film and growth environment make nucleation and high quality, impurity-free growth very difficult, thus limiting the applicability of this substrate. Limited process conditions are employed to grow on these substrates due to their high susceptibility to decomposition under H2. Non-uniform polarity of the substrate surface is also an issue, typically causing mixed polarity domains in the GaN epitaxial film. The fabrication of vertical devices structures on such substrates also involves issues of doping and suppression of decomposition.
HVPE (Halide Vapor Phase Epitaxy) GaN Substrates Via LILO (Laser Induced Lift-Off) and HVPE GaN Based FS GaN Substrates via Boule Growth
The HVPE GaN method is the most preferred method to date to produce FS GaN substrates. It enables large-area freestanding GaN wafers to be produced of high quality and low dislocation density, on which high quality, smooth epitaxial films and high quality devices can be fabricated. The process has the ability to be easily scaled to the desired size of the wafer, and substrate conductivity type can be readily controlled. Precursor and growth process set-up is relatively inexpensive compared to other techniques (e.g. high pressure crystal growth) and can be easily controlled with conventional process controls. Impurity incorporation is minimal and can be controlled through precursor purity and gas-phase ambient purity as well as reactor leak integrity and construction.
Homo-Epitaxial Growth on High Quality FS HVPE GaN Substrates
Because there have been no large area, freestanding GaN wafers commercially and readily available, there has been limited opportunity to develop the conditions to produce high quality epitaxial layer growth on FS GaN.
As discussed hereinafter in greater detail, the present invention enables growth of epitaxial films of crystalline quality at least as good as that of the substrate, resolving novel issues associated with the growth of epitaxy on FS GaN and other (Al,In,Ga)N FS substrates, and provides substantially improved device performance with epitaxy and devices characteristics that are superior to those on other conventional substrates such as sapphire.
The following discussion highlights some of the problems associated with homoepitaxy on FS GaN substrates, including problems observed in our initial epitaxial growth studies, which have been resolved by the present invention.
1) Morphology Smoothing and Pit-Filling
Initial studies on FS GaN “as grown” or unfinished substrates, shown in FIG. 1, using conditions for growth on sapphire, did not yield smooth epitaxial films after 2-3 microns of growth (see FIG. 2). It was determined that increased thickness of epi was required to make an improvement in the smoothing and pit filling of the surface morphology with standard growth conditions (as used for epi growth on sapphire or SiC substrates). A significant issue relating to growth on FS GaN “as grown” or unfinished surfaces is that appreciable MOVPE thickness has to be deposited to smooth out hillock morphology from the HVPE GaN. This is demonstrated in FIGS. 1-3.
Even at 7.5 μm of deposited epi, the film is still somewhat rough for MOVPE based device structures, as shown in FIG. 3. Pits in the substrate surface do not fill in readily under standard growth conditions and require extended growth times involving several microns of film growth to begin “smooth-out” of the surface. Growing thicker layers of epitaxy on the FS GaN substrate smoothes out the “as grown” or unfinished FS GaN morphology, but at the expense of increased cost and longer growth time for a device structure, which in turn increases the cost of the device structure growth and reduces the throughput and profitability of the vapor phase epitaxy (VPE) reactor operation.
2) Polishing Issues on FS GaN Substrates
As is the case with many other hard, brittle semiconductor crystals, such as SiC, polishing the GaN wafers prior to epitaxial growth surface is not trivial. Initial experiments involving polishing of FS GaN and subsequent MOVPE GaN growth revealed the occurrence of polishing scratches and poorly prepared surface using first cut conditions for polishing, shown in FIG. 4.
FIG. 5 shows the polishing induced damage in a 2.5 micron GaN epitaxial film grown on the substrate. There is reduced coherent growth and coalescence decorating what appears to be polishing damage or scratches.
Some smoother growth is revealed at higher magnification (255×) in FIG. 6, in areas where the 2.5 microns GaN epitaxial thin film grew, indicating that the film is attempting to grow in a two-dimensional fashion, replicating the underlying GaN substrate material.
3) FS GaN Backside Evaporative Products
Another issue related to homoepitaxial growth on FS GaN substrates is that the backside of the GaN wafer (N-face) tends to decompose during growth. This decomposition tends to interrupt the epi growth surface. Decomposition products escaping from the backside of the wafer are transported to the growth area disturbing the growth conditions and causing interrupted morphology.
FIG. 7 shows the normal morphology of a GaN PIN/10 μm GaN epi on FS GaN, while FIG. 8 shows the morphology degradation of an area where backside evaporative has inhibited the epitaxial layer growth. It is to be noted, however, that the backside evaporative product does not necessarily need to reach the epi surface in order to degrade the morphology, since the degradation product trapped between the susceptor and substrate material may modify the nature and extent of thermal contact, thereby introducing changes that may degrade the morphological uniformity of temperature-sensitive epitaxial layers and device structures.
The backside decomposition also changes the surface chemistry and therefore the nature of the electrical contacts formed on the backside of the gallium nitride substrate.
4) X-Ray FWHM Increase with DCXRD Slit Width
Another issue related to GaN substrates and epi thereon is the increase in GaN epi FWHM with increasing x-ray slit size, as shown in FIG. 9.
The DCXRD FWHM of the substrates increases with increasing x-ray slit width but at a lower rate than the 10 μm epitaxial layer and device structure thereon. This increased FWHM at larger slit width is attributed to bowing due to backside substrate evaporation, a thermal stress-related issue, epi tilt, and crystalline domains in the substrate. At smaller slit width the “as grown” FS GaN substrate and epitaxial layer have similar DCXRD FWHM.
5) Morphology Interruption in Epitaxial Films
The substrate preparation, interface preparation upon heat-up and substrate cleaning, also create issues in MOVPE GaN growth on FS GaN substrates. General reactor conditions may also affect epitaxial morphology for growth on FS GaN, resulting in poor interrupted morphologies. Proper coating on the susceptor and cleaning of the reactor parts are necessary to reduce contamination at the substrate epi interface. With other substrates such as sapphire and SiC, the cleanliness of the epitaxial growth system is less of an issue due to the highly defective interlayers that conventionally are grown on such substrates to reduce the lattice mis-match and strain between epi layer and substrate.
6) Contamination and Charge at Substrate-Epi Interface
Contamination at the homoepitaxial interface is defined here as any unintentional impurity defect or other flaw within 1000 Angstroms of the homoepitaxial epi and substrate interface and which has a concentration two times greater than the substrate or epi layer at a distance greater than 1000 Angstroms from the interface.
Potential impurity and structural damage at the interface leading to charge accumulation at the substrate-epi interface is a potential issue for the use of FS (Al, Ga, In)N in high frequency electronic devices. The homoepitaxial substrate—epi interface has substantial impurity concentrations deriving from cleaning, substrate preparation and reactor preparation conditions. High impurity concentrations (of species such as Si, O, C, S, etc.) at densities similar to those that might be found at epi-substrate interfaces usually result in interrupted epitaxial morphology unsuitable for epi device fabrication and production of high quality devices. FIG. 10 shows that Si=3E18 cm−3 and O=3.5E18 cm−3 at the interface with a S increase at the interface (1E16 cm−3).
All of the issues pertaining to contamination and charge at the substrate-epi interface (i.e. those discussed in Section 6) were identified from epitaxial growth on HVPE GaN substrate materials with the 10 micron HVPE GaN/sapphire structure, and have been demonstrated empirically. Further, these issues more generally affect all free-standing GaN and lattice-matched substrate generation and while specifically described hereafter with specific reference to HVPE GaN substrates, the resolution of such issues by the method of the present invention as hereinafter described is applicable to all FS GaN ((Al,In,Ga)N) or lattice-matched substrate generation.
Thus, initial investigations into the growth of GaN epitaxy on HVPE FS GaN (both “as grown” or unfinished and as polished or finished subsequent to growth formation) raise a number of questions, as set out below.                (1) Given the unavailability of GaN substrates, how can GaN of high crystalline quality (i.e. suitable for devices) be produced?        (2) How can high performance dislocation (low density) and improved material quality (smoothness, impurity level)-sensitive optoelectronic and electronic devices, e.g., ultraviolet light emitting diodes (UV LEDs), ultra-high-brightness blue LEDs, HEMTs (high electron mobility transistors), LDs (laser diodes) or PIN photovoltaic detectors be produced on FS GaN in a commercially reliable and reproducible manner?        (3) How can the GaN substrate be finished (e.g., by etch, polish, further processing, etc.) to yield good epitaxy?        (4) How can backside evaporation (or, more generally, evaporative products on (from) the backside of the wafer) during growth be inhibited?        (5) Is growth of epitaxy for devices on unpolished, FS GaN feasible?        (6) How can the as-grown or unfinished GaN surface be smoothed?        (7) How can the GaN substrate surface be smoothed most efficiently and cost-effectively to allow FS GaN substrates to be a preferred substrate of choice for all GaN epitaxial applications?        (8) What are optimal substrate cleaning conditions, heating conditions and reactor preparation conditions for producing good epitaxy on FS GaN?        (9) How can the HVPE substrate be cleaned to produce a surface suitable for epitaxial growth?        (10) What crystallographic orientation yields the best epitaxy for devices and what are the growth conditions to produce this epitaxy?        (11) How can potential tilt, mis-oriented crystal grains, inversion domains and other crystallographic defects be overcome to achieve high quality epitaxial growth on FS GaN?        (12) How can substrate-epi interface contamination and unintentional charge build up (n or p) be avoided, eliminated, or negated (to produce no resultant unintentional charge)?        (13) How can the GaN substrate material and high quality strain-free epi thereon enable novel device structures deemed less desirable on other substrates?        
The art has not satisfactorily resolved these issues, which are addressed by the present invention.